1. Field of the Invention
The present invention provides a photoelectron linear accelerator for producing a polarized electron beam with low emittance.
2. Description of the Prior Art
Polarized electron beams are a principal investigative tool at a number of major accelerator centers. It has been demonstrated that polarized electrons will be extremely useful in electron position colliders. Current polarized electron beams for accelerators are generated by dc-biased electron guns that utilize gallium arsenide (GaAs) as the photocathode material. The relatively long pulse (on the order of nanoseconds) generated by these sources is rf chopped and bunched in the injector to derive the desired pulse structure, including microbunch number and temporal width, to match the accelerator and experiment requirements.
The normalized rms transverse eminance of high charge rf-bunched beams is typically on the order of 10xe2x88x924 m. Future colliders require an emittance of xcx9c10xe2x88x928 m in at least one plane. Current designs achieve this extremely low emittance in the vertical plane using an appropriately designed damping ring. Since the photoemitted electrons are rapidly accelerated to relativistic energies by electric fields that are much higher than used in dc guns, the effects of space charge on emittance growth are minimized. Since the initial emittance growth in an rf gun is correlated, this growth can be reversed by placing a solenoidal field immediately after the cathode. An emittance-compensated, rf photoinjector is normally designed to achieve the minimum emittance at a compensation point some distance beyond the solenoid exit. Simulations indicate that emittances as low as 10xe2x88x926 m for 1nC of charge per micropulse can be achieved with an rf photoinjector for round beams, although the measured values tend to be slightly larger.
Photoinjectors are currently in widespread use and have been proposed as a source of cw unpolarized electron beams for energy recovery linacs (ERL). The gun laser required for an ERL may only be feasible if a GaAs (visible laser) or CsK2Sb (green) cathode is utilized. In this case, the plane wave transformer (PWT) injector would have to provide adequate cooling. The cooling requirement is somewhat less stringent in some versions of electron ion colliders, which require polarized electrons, for which the rf frequency of the cw injector can be quite low.
The problem for a dc gun is not the gradient on the cathode, which can be fairly high and potentially even as high as the field on the cathode of a PWT gun at extraction. Thus the emittance of the beam exiting a dc gun can be comparable to that exiting an rf gun, but the energy is 5 to 50 times lower. If a short pulse high-charge beam is required, as for a collider, the problem is coupling the still low-energy beam to an accelerating structure before the emittance (both the transverse and especially the longitudinal emittance) grows significantly due to the intense space charge forces. Emittance compensation should in principle work for a dc gun as well as an rf gun, but the problem is the vastly lower energy and thus the effect of the space charge field still remains.
What is thus desired is to provide a device for providing a polarized electron beam using an rf gun, the beam having a low emittance.
The present invention provides a method and apparatus to produce a high-quality, polarized electron beam and, in particular, uses an rf photoelectron gun using the PWT photoelectron linear accelerator design, thereby generating a lower emittance beam than available in the prior art.
Semiconductors such as binary compounds (and their ternary and quartemary analogs) combining elements from the III and IV columns of the periodic table, for example, gallium arsenide, are proven cathode materials which are used to produce polarized electron beams. A polarized electron beam is produced when such a cathode semiconductor is illuminated by a circularly polarized laser beam. An ultra high vacuum ( less than 10xe2x88x9211 Torr) condition is provided in order for the semiconductor target to have good quantum efficiency and long lifetime for the production of polarized electrons.
The present invention utilizes certain features of conventional dc-biased polarized guns to produce polarized electron beams using an rf gun, in order to dramatically improve the emittance of the beam. A low emittance is desired and is an indication of the good quality of the electron beam.
The PWT rf gun design is especially well matched to the features necessary for production of polarized electrons. Specifically, the PWT design has 1) an inherently high vacuum conductance which improves the vacuum, 2) an integrated photocathode inside an rf linear accelerator, and 3) an emmitance compensating beam focusing system which improves the beam quality.
Additional features that further improve the operation of the PWT gun for the production of a polarized electron beam include a load-lock for introducing the activated semiconductor coated cathode under ultra-high vacuum conditions into the PWT tube structure, enhancing the inherently superior vacuum pumping potential of the PWT design by enlarging the diameter of the outer cylinder, and coating the interior cylindrical tube wall with a thin-film of residual gas absorbent such as TiZrV.
The present invention thus provides an improved rf photoelectron gun for producing a polarized electron beam with low emittance.